The field of the invention is alarm sirens for use primarily in domestic installations where the siren is to be driven by a fractional horsepower motor. The most important example of this is in vehicle alarms which operate of 12 volt systems.
Alarms of this type are known generally and comprise relatively simple structures. Usually there is a framework or bracket which mounts the motor and may have electrical terminals, switches and the like also mounted thereon which are of no concern to this invention. The shaft of the motor protrudes through a support plate, and a casing or housing which is generally cup-shaped is mounted on the support plate coaxially with the shaft. Means are normally provided for accurately centering the casing or housing which will be called a stator hereinafter. The outer flange or the stator has a plurality of perforations to cooperate with similar perforations provided in a rotor that is mounted on the shaft. The rotor is of a configuration closely following that of the stator -13 hence cup-shaped and in addition to the perforations on its cylindrical outer flange, it has vanes to promote movement of air in creating the turbulence that produces the high pitched sound desired.
The invention is primarily concerned with small sirens, say of a diameter of about 3 inches or so, which can be driven at speeds of 10 to 13,000 r.p.m. and more for producing very high-pitched tones. It is not so limited, however, although as will be seen, the major advantages come from its use in these small sirens.
Automobile alarms, for example, are intended to be installed quickly and economically and readily moved from vehicle to vehicle by the owner. The expense involved becomes an important factor in the business of making and installing such alarms, but it is axiomatic that function is the most important criterion. It becomes a compromise between economy and performance which will determine the quality of a given installation.
The low-priced sirens which are available today, utilize generally two types of rotors, the rotor being the key to the performance of the siren. So far as known, these types are die-cast rotors and fabricated rotors.
The die-cast rotors are subject to flaws during casting, are necessarily formed with thick parts for strength and are expensive. The difficulties of casting absolutely uniform and symmetrical rotors means that these rotors are often not perfectly balanced and will not rotate as fast as they could if they were perfectly balanced. Unbalanced rotors use more power than balanced rotors.
The fabricated rotors are made by assembling at least two parts which have been separately formed. These is a cup-shaped part which has a base plate and a plurality of rectangular perforations around the periphery of the flange, and there is a vane part which has a central support and arms with vanes punched and formed thereon. These two parts are spot-welded or staked together. There is a problem of achieving perfect balance here.
The invention uses a rotor which is a unitary article formed from a single piece of sheet metal and is perfectly balanced and much more economical to manufacture than either of the other two types of rotor. It is lighter in weight than either, it is capable of higher speeds and will rotate faster for the same motor and produce a louder sound. It eliminates considerable assembly time.